1. Field of the Invention
This invention relates to loudspeakers and in particular to loudspeakers with improved dynamic range as compared to existing loudspeakers.
2. Discussion of Prior Art
Conventional (or single input) loudspeaker systems can be defined as systems in which the master drive signal may be passed to a plurality of drivers, but for which, at any particular frequency the relationship between the signals passed to each driver is fixed. A driver in this context could mean an electro-magnetic induction coil (as used in conventional loudspeakers) or a piezo-electric pad or any other device that can cause a panel-form loudspeaker or a loudspeaker cone to move.
FIG. 1 shows a conventional loudspeaker system comprising three drivers/loudspeakers 1, 2, 3. A master signal 4 is split by filters 5, 6 and 7 (high pass filter, band pass filter and low pass filter respectively) into three frequency ranges, treble 5a which goes to speaker 1, mid-range 6a which goes to speaker 2 and bass 7a which goes to speaker 3. This represents a multiple speaker system in which there is a frequency split of the main master drive signal 4. The relationship between each of the drivers 1, 2 and 3 is fixed and is not dependent on the level of the master signal.
Conventional analogue loudspeakers have a limited dynamic range as compared to the available dynamic range of the latest digital recordings (for example 24 bit or DSD). Digital recordings use up to 24 bits and this implies a dynamic range of 141 dB. Digital loudspeakers, involving 2N single bit devices (with N=24, this number is 1.7×107) have been proposed—see WO96/31086. However, these suffer from obvious complexity and poor performance associated with the interaction effects between the different devices, which have discouraged widespread use of such systems. A further problem is the inability of most loudspeakers to reproduce realistic absolute levels of sound (up to say 120 dB at 1 m without distortion), so such digital loudspeakers cannot take full advantage of the 24-bit fidelity.
A conventional loudspeaker system, such as that shown in FIG. 1, will suffer distortion and other detrimental effects if the dynamic range supplied to any of the drivers/loudspeakers 1, 2 or 3 exceeds much more than 100 dB. Note, although conventional speakers can be constructed to have a dynamic range of approaching 120 dB they are very expensive. More usually the dynamic range of a conventional speaker is in the region of 100 dB.